Export Blender 2.79 PBR shader to Unreal Physical based material
Bit of a late answer, but I have recently released an add-on for Blender 2.8 to do exactly this simply and easily (though admittedly not quickly - baking in Blender, like rendering, takes time).

Documentation here: http://www.toohey.co.uk/SimpleBake/

Available here: https://gumroad.com/l/mzYoz

It is also possible to do this manually. In a nutshell, you need to create an image within Blender (in the UV Image Editor) and add an image node to every material you are using set to that image. That image node needs to be selected in every material, though it doesn't need to be "hooked up" to anything at all. It just floats there.

The best way is to then take each input to your Principled BSDF shader one by one, run it through an Emission shader (connected to the Material Output in place of the Principled BSDF you had before) and then bake emission.

It's a bit of a long and painful process if you have an object with lots of materials - hence why I wrote the add-on.